Circuit arrangement for a high frequency furnace



April 11, 1961 L. BLOK ETAL CIRCUIT ARRANGEMENT FOR A HIGI-I FREQUENCY FURNACE Filed June 1, 1956 2 Sheets-Sheet 1 INVENTOR LOURENS BLOK JACOB WILLEM DE RUITER SIJTSE JOHANNES CORNgLlS BY YTSMA GOO ZEWANENBURG an; 4.

AGE T Aprll 11, 1961 BLQK ETAL 2,979,669

CIRCUIT ARRANGEMENT FOR A HIGH FREQUENCY FURNACE Filed June 1, 1956 2 Sheets-Sheet 2 INVENTOR LOURENS BLO K LEM DE RUITER YI HANNES CORMELISA SYTSM BY 600 EN ZWAN'ENBURG United States Patent '0 CIRCUIT ARRANGEMENT FOR A HIGH FREQUENCY FURNACE Lourens Blok, Jacob Willem De Ruiter, Sijtze Johannes Cornelis Sytsma, and Gooitzen Zwanenburg, all of Eindhoven, Netherlands, assignors, by mesne assignments, to North American Philips Company, Inc., New York, N.Y., a corporation of Delaware Filed June 1, 1956, Ser. No. 588,899

Claims priority, application Netherlands June 3, 1955 8 Claims. (Cl. 331 62) The present invention relates to high frequency furnace circuit arrangements. More particularly, the invention relates to high-frequency furnaces of high power, for example of l kilowatt or more, which are provided with a grid-controlled output tube having a resonant circuit connected in the anode circuit, the coil of the resonant circuit being coupled to a coupling coil connected in the load circuit. The anode voltage of the output tube is taken from an anode voltage supply source one end of which is connected to ground.

High-frequency furnaces of this kind are used in practice both for dielectric and for inductive heating.

In such high-frequency furnaces which may have a power of, for example, from to kilowatts, considerable leakage losses occur at the output transformer constituted by the resonant circuit coil and the coupling coil.

It is an object of the invention to provide a special embodiment of such a high-frequency furnace with consider-'- ably reduced leakage losses.

According to the Invention, in a high-frequency fur nace of the kind described in the preamble, the resonant circuit is provided with a tapping point connected to ground. The anode and cathode of the tube are connected each to one of two points of the resonant circuit situated on either side of the tapping point; said points being each at approximately one half of the circuit alternating voltage. Also, both the anode and the cathode are connected to the anode voltage supply source through a high-frequency choke coil.

In the circuit arrangements of the present invention, the alternating voltage produced at the circuit ends with respect to ground is approximately halved; hence, the output transformers have to satisfy reduced requirements only with respect to insulation and this permits not only the use of a cheaper construction, but also of a smaller spacing between the resonant-circuit coil and the coupling coil, which enables the leakage losses to be materially reduced in practice.

In a structurally advantageous embodiment of the highfrequency furnace in accordance with the invention, the grounded tapping point on the resonant circuit is the grounded tapping point on a capacitive voltage divider connected between the circuit ends.

In order that the invent'on may be readily carried into efiect, it will now be described in detail with reference to the accompany drawing, wherein:

Fig. 1 is a schematic diagram of an embodiment of circuit arrangement of a known high-frequency furnace;

Fig. 2 is a schematic diagram of an embodiment of a circuit arrangement of a high-frequency furnace in accordance with the invention;

Fig. 3 is a modification of the circuit arrangement of Fig. 2; and

Fig. 4 is a schematic diagram of a preferred embodiment" of a circuit arrangement of a high frequency furnace of the present invention.

M I ice In the circuit arrangement of Fig. 1, the high-frequency heating energy is taken from an electron-tube oscillator comprising a triode 1. The tuned anode circuit of the oscillator, which is connected in a Colpitts circuit, is provided with a resonant circuit 2 which determines the oscillator frequency and comprises a coil 3 and a circuit capacity constituted by a capacitive voltage divider comprising a capacitor 4 and a three-electrode capacItor 5 connected in series therewith. The anode of the triode 1 is coupled through a blocking capacitor 6 to that end of the capacitor 4 which is connected to the resonant-circuit coil 3 and also, through a high-frequency choke 7, to an anode-voltage supply source 8, one end of which is connected to ground and which is shunted by a smoothing capacitor 9. Through a grid capacitor 10 and a leakage resistor 11, the control grid of the triode 1 is connected to a tapping point on the capacitor 5, which tapping point is constituted by the middle one of the three electrodes. The grounded tube cathode is connected to a junction of the capacitors 4 and 5, which junct'on is approximately the electrical middle of the resonant circuit 2.

When the circuit arrangement oscillates, an alternating voltage which is symmetrical with respect to ground is produced across the resonant circuit coil 3; one half of the circuit-voltage Is approximately equal to the anode direct-voltage. Through a coupling coil 12 the output power is supplied to a load circuit 13. For this purpose, a working coil 15 is connected to the output terminals 14, 14' of the coupling coil 12, in which working coll a workpiece to be heated is arranged.

It was found in practice that, in this known embodi- 7 ment of a high-frequency furnace, very inconvenient leakage losses occur, particularly when the output transformer is designed to provide variable coupling, as is usual to achieve load matching.

A high-frequency furnace in accordance with the invention will now be described in detail with reference to Fig. 2. In Fig. 2, parts similar to those in Fig. l are designated by like reference numerals.

The high-frequency furnace shown in Fig. 2 contains a directly heated triode 1 which is supplied from an anode voltage source 8 which is shunted by a smoothing capacitor 9. The cathode-filament current for the tube is taken from an alternating voltage supply source (not shown) and supplied to the tube filament through input terminals 17, 17' and a filament current transformer 18.

The triode 1 is again connected as a Colpitts oscillator; the anode circuit contains an anode resonant circuit 19 having a coil 20 which is shunted by a capacitive voltage divider comprising capacitors 21 and 22 which form part of the circuit capacitance. The voltage produced across the capacitor 22 is supplied as a feedback voltage, through a grid capacitor 10, to the control-grid of the tube. Similarly to the circuit arrangement of Fig. 1, the anode circuit is inductively coupled to a load circuit 13, through a coupling coil 12, .which, together with the resonant-circuit coil 20, constitutes an output transformer.

According to the invention, in order to reduce to a minimum the requirements to be satisfied by the output transformer 20, 13' with respect to insulation, and consequently the leakage losses, the resonant circuit is provided with a tapping point connected to ground. The anode and cathode of the tube areconnected each to one of two points of the resonant circuit situated on either side of the grounded tapping point, which points are each at about one half of the circuit alternating voltage. In addition, both the anode and the cathode of the oscillator tube are connected to the anode voltage I supply source through a high-frequency choke coil. In

estates the embodiment of Fig. 2, the coil 20 is provided with a grounded center tapping point 23. The anode of the triode 1 is coupled, through a blocking capacitor 6, to the junction of the capacitor 21 and the coil 20 and also, through the high-frequency choke 7, to the anode voltage Supply source 8. The cathode of the triode 1 is connected to the tapping point on the voltage divTder 21, 22 and also, through filament-current leads 24, 24' having chokes 25 and 25 connected in them, to the secondary winding 26 of the filament current transformer 18. The transformer 18 is shunted by two series-connected capacitors 27, 27, the junction of which is connected to ground. The grounded connecting terminal of the anode voltage supply source 8 is connected to the middle, of the secondary'winding 26 of the filament current transformer 18.

The high-frequency alternating voltages which, in this circuit arrangement, are produced across the part of the resonant circuit connected between the anode and the cathode, similarly to the circuit arrangement of Fig. 1, are approximately equal in amplitude to the anode direct voltage. However, in contradistinction to known circuit arrangements, both the anode and the cathode of the triode 1 are connected, through chokes, to the grounded anode voltage supply source, the anode circuit being provided with a grounded center tap, so that both circuit ends are only at half the circuit alternating voltage with respect to ground. If, for example. the triode 1 has an anode direct voltage of about 12 kilovolts suppliedto it, with an alternating voltage of, for example, 1 kilovolt set up across the capacitor 22, the total circuit voltage may be about 13 kilovolts. The alternating voltage at the circuit ends, however, in this case is only about 6 /2 kilovolts with respect to ground,

due. to the fact that. the center of the coil 20 is connected to ground. I v

In other words, the voltage between the circuit-coil and the coupling coil is about halved as compared with the known circuit arrangements, so that a corresponding reduction of the insulation requirements is achieved. The circuit-coil to coupling-coil spacing can be made considerably smaller in order to reduce the leakage losses of the output transformer. I

In order to provide simple operation, the high-frequency furnace shown in Fig. 2 is provided with a startstop switch 28 connected in the control-grid circuit. In the position shown, the switch 28 connects the conrolgrid of the triode 1, through the series combination of a, high-frequency choke coil 29 and a grid resistor ll, to ground, so that oscillating is produced. The grid resistor 11 is shunted by a bypassing capacitor 30 in order to bypass it with respect to high-frequency voltages.

When the start-stop switch 28 is switched over into the position different from that shown, the control-grid circuit is connected to a negative grid-bias source 31, so that the triode 1 is cut ofi.

Fig. 3 is an alternative embodiment of the high-frequency furnace of Fig. 2. Similar elements are again designated by like reference numerals.

Similarly to the circuit arrangement of Fig. 2, the embodiment shown in Fig. 3 contains a directly heated triode 1 connected as an oscillator. The anode circuit of the triode 1 is provided with a resonant circuit 32 comprising a coil 33 which, together with thecoupling coil 12 connected in the load circuit 13, constitutes an output transformer which is made variable for load matching. The resonant circuit 32 is again provided with a grounded tapping point. In contradistinction to the circuit arrangement of Fig. 2, the grounded tap ping point on the anode circuit, however, is constituted by the grounded tapping point on a capacitive voltage divider which is connected between the circuit-ends and comprises the series-combination of capacitors 34, 35 and 22. The voltage-divider capacitors 34 and 35 are con-f nected-between the anode and the cathode to replace-the voltage-divider capacitor 21 of Fig. 2. The junction of the capacitors 34 and 35 is, connected to ground. This manner of connecting the resonant circuit to ground is constructionally attractive and also has the advantage that the variable output'transformer can be designed as is described in United States Patent No. 2,662,162, issued December 8, 1953 to Blok, whereby the circuit-coil is axially movablein the coupling coil.

A further. difference in the embodiment of Fig. 3 from the circuit arrangement of Fig. 2 consists in the filamentcurrent circuit of the triode 1.

Through the filament-current leads 24, 24, the cathode filament is directly connected to the secondary 26 of the filament-current transformer 18, the cathode-end which is connected to the resonant circuit being connected, through a high-frequency choke 36, to the grounded terminal of the anode voltage supply source. 8.

The primary winding 37 of the filament-current transformer 18 is, connected, through chokes 38, 38, to the input terminals 17, 17' which are also connected to an alt mating voltage supply source (not shown).

The primary transformer circuit is symmetrically grounded through the coupling capacitors 39, 39' connected to input terminals 17 and 17', respectively.

In order to restrict the high-frequency voltages induced in the primary 37 and the iron core of the filament-current transformer 18, the transformer windings 26, 37 are inter-connected through two capacitors 40, 40', through which the ends of the primary 37 and the secondary 26-. respectively, are inter-connected.

In the design of the filament-current circuit, the chokes 38, 38 carry the primary current of the filament-current transformer, which current is much smaller than the filamerit-current itself. This provides the advantage that the chokes 38, 38 may be proportioned comparatively small in spite of the large filament currents (about 100 I amperes) used for high power tubes.

The circuit arrangement operates in a manner which is substantially similar to that described with reference to Fig. 2; in addition, it may be observed that a suitable choice of the capacitors of the voltage divider 34, 35, 22 connected between the circuit ends permits a symmetrical voltage loading of the circuit with respect to ground.

7 Fig. 4 is a modification of the embodiment of 3 which has proved of advantage in practice. Similar elements are again designated by like reference numerals. In the embodiment of Fig. 4, the resonant circuit 41, which is connected in the anode circuit of the triode 1 and isturrd to the operating frequency, similarly to the circuit arrangement shown in Fig. 3, is grounded through the grounded tapping point on a capacitive voltage divider connected between the circuit ends.

In Fig. 4, the capacitive voltage divider comprises the series combination of four capacitors 42, 43, 44 and 22. Voltage-divider capacitors 43 and 44 have equal capacitance values and are connected between the anode and cathode of the triode 1. The junction of these capacitors is connected to ground. The capacitor 42 is connected betweenthe anode and the adjacent circuit end and its capacitance value is equal to that of the voltage-divider capacitor 22 which is connected between the cathode and the circuit end adjacent the cathode. Thus, the grounded tapping point on the voltage divider 42, 43, 44, 22 is the electrical center of the circuit, so that not only the alternating voltages produced at the anode and the cathode but also those set up at the circuit ends are symmetrical with respect to ground. If again across the triode 1 an anode, alternating voltage of 12 kilovolts is produced, the alternating voltages appearing at the anode and the cathode are each 6 kilovolts with respect to ground and balanced with respect to one another; the alternating voltages produced at the circuit-ends, in this event, are, for example, 7 kilovolts with respect to ground.

a 'Th'efilament-current supply circuit is distinguished from that used in the embodiments described hereinbefore by field produced by the high-frequency voltages set up across the secondary 46 is limited substantially entirely by the screening cylinder 48 and the fianges47, so that the primary 49 and the iron core of the filament-current transformer 45 are efliciently screened. Thus, the primary 49 is directly connected, without the interposition of choke .coils, to the alternating voltage supply source (not shown) from which the filament-current supply is derived. The cathode-lead choke, similarly to the embodiment of Fig. 3, is connected to one of the cathode ends.

It will be evident that the various filament-current supply circuits described in the embodiments shown in Figs. 2, 3 and 4 may be int rchanged.

In some cases, it will be of advantage constructionally to shunt the circuit-coil by an additional capacitor 50, which is shown in broken lines in Figs. 2, 3 and 4. Thus, it is ensured that the capacitive voltage divider connected between the circuit ends carries only part of the total circuit current, which permits the use of capacitors having air as a dielectric; this is of advantage with respect to manufacturing costs.

Finally it should be noted, that the invention is not restricted to the use of the oscillator provided with capacitive feedback which is shown in the various embodiments, but that obviously the invention can also be used, when the triode 1 is provided with inductive feedback or when the triode 1 is excited by a preceding tube.

While the invention has been described by means of specific examples and in specific embodiments, we do not wish to be limited thereto, for obvious modifications will occur to those skilled in the art without departing from the spirit and scope of the invention.

What is claimed is:

l. A circuit arrangement for a high frequency furnace, comprising an electron discharge tube having an anode, a cathode and at least one grid, an output circuit connected in the anode circuit of said tube, said output circuit comprising a resonant circuit having a resonant coil and a shunt circuit connected across said resonant coil, a capacitor, means connecting said anode through said capacitor to a first point in said output circuit, means connecting said cathode to a second point in said output circuit, means connecting said grid to a third point in said output circuit thereby to control the grid excitation voltage, a tapping point in said output circuit interposed between said first and second points, said tapping point being connected to a point at ground potential and being connected substantially symmetrically with respect to the ends of said resonant coil and being so positioned that the voltage between each of said first and second points and, said tapping point is substantially half the voltage produced by said circuit arrangement, a load circuit including a coupling coil inductively coupled to said resonant coil whereby a substantially reduced voltage appears between said resonant coil and said coupling coil, a source of voltage supply having a pair of terminals, one of the terminals of said supply source being connected to a point at ground potential, a first high frequency chokecoil, means connecting said anode to one terminal of said supply source through said first high frequency choke coil, a second high frequency choke coil, and means connecting said cathode to the other terminal of said supply source through said second high frequency choke coil.

2. A circuit arrangement for a high frequency fur nace, comprising an electron discharge tube having an anode, a cathode and a grid, a resonant circuit con? nected in the anode circuit of said tube, said resonant circuit comprising a resonant coil, a shunt circuit connected across said resonant coil, a capacitor, means conmeeting said anode through said capacitor to a first point in said shunt circuit, means connecting said cathode to a second point in said shunt circuit, a tapping point on said resonant coil interposed between said first and second points substantially symmetrically with respect to the ends of said resonant coil, said tapping point being connected to a point at ground potential and being so positioned that the voltage between each of said first and second points and said tapping point is substantially half the voltage produced by said circuit arrangement, a load circuit including a coupling coil inductively coupled to said resonant coil whereby a substantially reduced voltage appears between said resonant coil and said coupling coil, means connecting said grid to a third point in said shunt circuit thereby to control the grid excitation voltage, a source-of voltage supply having a pair of terminals, one of the terminals of said supply source being connected to a point at ground potential, a first high frequency choke coil, means connecting said anode to one terminal of said supply source through said first high frequency choke coil, a second high frequency choke coil, and means connecting said cathode to the other terminal of said supply source through said second high frequency choke coil.

3. A circuit arrangement for a high frequency furnace comprising an electron discharge tube having an anode, a cathode and a grid, a resonant circuit connected in the anode circuit of said tube, said resonant circuit comprising a resonant coil, a shunt circuit connected across said resonant coil, a capacitor, means connecting said anode through said capacitor to a first point in said shunt circuit, means connecting said cathode to a second point in said shunt circuit, a tapping point on said resonant coil interposed between said first and second points substantially symmetrically with respect to the ends of said resonant coil, said tapping point being connected to a point at ground potential and being so positioned that the voltage between each of said first and second points and said tapping point is substantially half the voltage produced by said circuit arrangement, a load circuit including a coupling coil inductively coupled to said resonant coil whereby a substantially reduced voltage appears between said resonant coil and said coupling coil, means connecting said grid to a third point in said shunt circuit thereby to control the grid excitation voltage, a source of voltage supply having a pair of terminals, one of the terminals of said supply source being connected to a point at ground potential, current supply means for said cathode comprising a current transformer having a primary winding, a secondary winding, a pair of high frequency choke coils, said choke coils being connected between the terminals vof said secondary winding and the terminals of said cathode, means connecting said cathode to one terminal of said supply source through said pair of choke coils and means for applying an alternating voltage to said pri mary winding, a high frequency choke coil, and means connecting said anode to the other terminal of saidsupply source through said last-mentioned high frequency choke coil.

connecting said cathode'to a second point in said shunt ace-aces circuit, means connecting said grid to a third point in said shunt circuit thereby to control the grid excitation volt-age, a tapping point on said voltage dividerinterposedbetween said first and second points, said tapping point being connected to a point at ground potential and being so positioned that'the voltage between each of said .first and second points and said tapping point is substantially half the voltage produced by said circuit arrangement, a load circuit including a coupling coil variably inductively coupled to said resonant coil whereby a substantially reduced voltage appears between said resonant coil and said coupling coil, a source of voltage supply having a pair of terminals, one of the terminals of said supply source being connected to a point at ground potential, at first high frequency choke coil, means connecting said anode to one terminal of said supply source through said first high frequency choke coil, a second high frequency choke coil, and means connecting saidcathode to the other terminal of said supply source through said second high frequency choke coil.

5. A circuit arrangement for a high frequency furnace, comprising an electron discharge tube having an anode, a cathode and a grid, a resonant circuit connected in the anode circuit of said tube, said resonant circuit comprising a resonant coil, a shunt circuit connected across said resonant coil, said shunt circuit comprising at capacitive voltage divider, said voltage divider'comprising at least two capacitors connected in series circuit arrangement, means connecting said anode to a first point in said shunt circuit on one side of two of said capacitors, means connecting said cathode to a second point in said shunt circuit on the other side'of said two capacitors, a tapping point on said voltage divider between said two capacitors, said tapping point being connected to a point atground potential and being so positioned that the voltagefbetween each of said first and second points and said tapping point is substantially half the voltage produced by said circuit arrangement, a load circuit including a coupling coil variably inductively coupled to said resonant coil whereby a substantially reduced voltage appears between said resonant coil and said coupling coil, a source of voltage supply having a pair of terminals, one of the terminals of' said supply source being connected to a point at ground potential, means for applying a control voltage to said grid comprising a third capacitor connected in said shunt circuit between said cathode and the said grid, at first high frequency choke coil, means connecting said anode to one terminal of said supply source through said first high frequency choke coil, a second high frequency choke coil and means connecting said cathode to the other terminal of said supply source through said second high frequency choke coil,

6. A circuit arrangement for a high frequency furnace, comprising an electron discharge tube having an anode, a cathode and a grid, a resonant circuit connected in the anode circuit of said tube, said resonant circuit comprising a resonant coil, a shunt circuit connected across said resonant coil, said shunt circuit comprising a capacitive voltage divider, said voltage divider comprising a plurality of capacitors connected in series circuit arrangement, means connecting said anode to a first point in said shunt circuit on one side of two of said capacitors, means connecting said cathode to, a second point in said shunt circuit on the other side of said two capacitors, a tapping point on said voltage divider between said two capacitors, said tapping point being connected to a point at ground potential and being so positioned that the voltagebetween each of said first and second points and said tapping point is substantially half the voltage producedby said circuit arrangement, a load circuit including at coupling coil variably inductivelycoupled to said resonapt; coilwhereby, a substantially reduced voltage apb tw e t aiq srscna t a audrsaid up g coil,

a source of voltage supply having a pair of terminals, one of the terminals of said supply source being connected to a point at ground potential, means for applying a control voltage to said grid comprising athird capacitor connected in said shunt circuit between said cathode and the said grid, current supply means for said cathode comprising a current transformer having a primary winding, a secondary winding, a pair of choke coils, means for applying an alternating voltage to said primary winding, said choke coils being connected between the terminals of said primary winding and said means for applying an alternating voltage, and means directly connecting the terminals of said cathode to the terminals of said secondary winding, a first high frequency choke coil, means connecting said anode to one terminal of said supply source through said first high frequency choke coil, a second high frequency choke coil and means connecting said cathode to the other terminal of said supply source through said second high frequency choke coil.

7. A circuit arrangement for a high frequency furnace, comprising an electron discharge tube having an anode, a cathode and a grid, a resonant circuit connected in the anode circuit of said tube, said resonant circuit comprising a resonant coil, a shunt circuit connected across said resonant coil, said shunt circuit comprising a capacitive voltage divider, said voltage divider comprising a plurality of capacitors connected in series circuit arrangement, at least two of said capacitors having equal capacitance values, means connecting said anode to a first point in said shunt circuit between a third of said capacitors and said two capacitors, means connecting said cathode to, a point in said shunt circuit between a fourth of said capacitors and said two capacitors, said third and fourth capacitors having equal capacitance values, a tapping point on said voltage divider between said two capacitors, said tapping point being connected to a point at ground potential and being so positioned that the voltage between each of said first and second points and said tapping point is substantially half the voltage produced by said circuit arrangement, a load circuit including a coupling coil variably inductively coupled to said resonant coil whereby a substantially reduced voltage appears between said resonant coil and said coupling coil, a source of voltage supply having a pair of terminals, one of the terminals of said supply source being connected to a point at ground potential, means for applying a control voltage to said grid comprising said fourth capacitor connected between said cathode and the said grid, a first high frequency choke coil, means connecting said anode to one terminal of said supply source through said first high frequency choke coil, a second high frequency choke coil, and means connecting said cathode to the other terminal of said supply source through said second high frequency choke coil.

8. A circuit arrangement for a high frequency furnace, comprising an electron discharge tube having an anode, a cathode and a grid, a resonant circuit connected in the anode circuit of said tube, said resonant circuit comprising a resonant coil, a shunt circuit connected across said resonant coil, said shunt circuit comprising a capacitive voltage divider, said voltage divider comprising a plurality of capacitors connected in series circuit arrangement, two of said capacitors having equal capacitance values, means connecting said anode to a first point in said shunt circuit between a third of said capacitors and said two capacitors, means connecting said cathode to a second point in said shunt circuit between a fourth of said'capacitors and said two capacitors, said third and fourth capacitors having equal capacitance values, a tapping point on said voltage divider between said two capacitors, said tapping point being connected to a point at; ground potential and being so positioned that the voltagebetween eachvoft said first and second point:

and said tapping point is substantially half the voltage produced by said circuit arrangement, a load circuit including a coupling coil variably inductively coupled to said resonant coil whereby a substantially reduced voltage appears between said resonant coil and said coupling coil, a source of voltage supply having a pair of terminals, one of the terminals of said supply source being connected to a point at ground potential, means for applying a control voltage to said grid comprising said fourth capacitor connected between said cathode and the said grid, current supply means for said cathode comprising a current transformer having a primary winding, a metal shield connected to a point at ground potential and having a substantially cylindrical configuration and protruding flange arms thereon, a secondary winding positioned in said shield, means for applying an alternating voltage to said primary winding and means directly connecting the terminals of said cathode to the terminals of said secondary winding, a first high frequency choke coil, means connecting said anode to one terminal of said supply source through said first high frequency choke coil, a second high frequency choke coil, and means connecting said cathode to the other terminal of said supply source through said second high frequency choke coil.

References Cited in the file of this patent UNITED STATES PATENTS 1,739,299 Gerth et a1. Dec. 10, 1929 1,755,386 Chireix Apr. 22, 1930 2,145,124 Mead Jan. 24, 1939 2,446,032 Watts et a1. July 27, 1948 2,579,525 Varela Dec. 25, 1951 

